The experiences in various communities in the United States have been not unlike those described for European cities. Abbott in describing the successive epidemics in Massachusetts remarked that the 1889–90 spread manifested itself by a sudden rise in the mortality from influenza and pneumonia, beginning about December 20th and culminating in the middle week of January, thereafter falling off quite suddenly in February to about the usual rate for these diseases. The second epidemic two years later began with a more gradual rise in October and November and then increased sharply in December, continued for nearly three weeks at its maximum in January, and declined nearly as sharply as in the previous epidemic two years before.
Winslow and Rogers who have studied the 1918 epidemic as it affected the various towns of Connecticut observed that the outbreak in a given community generally occupied a period of from six to eight weeks, and was steep and abrupt in communities which were badly hit, flatter and more gently sloping in those which escaped lightly. Also the outbreak was more severe in communities receiving the infection early than in those later affected.
Mortality curves.—Pearl has studied the epidemic constitution of influenza in forty-two of the large cities of the United States. He has plotted the annual death rate per 1,000 population from all causes in each week, from the week ended July 6, 1918, up to January 1, 1919, and observed a very distinct difference in the type of curve for deaths from all causes during the epidemic period in the various cities. These differences have been chiefly in respect to the severity and suddenness with which they were attacked. Thus Albany, Boston, Baltimore, Dayton and Philadelphia show an initial explosive outbreak of great force, while Atlanta, Indianapolis, Grand Rapids, Milwaukee and Minneapolis exhibit a much slower and milder increase of the mortality rate. In Albany and Baltimore the curve of the first epidemic outbreak rises to a peak and declines at about the same rate. In Cleveland and St. Paul, on the other hand, the rate of ascent to the peak is very rapid, while the decline is slow and long drawn out.
Some of the cities, such as Albany, show but a single well defined peak in the mortality curve. Others, such as Boston, New Orleans and San Francisco show two peaks; while still others, like Louisville, show three well marked peaks.
Usually the first was the highest and the second and third were progressively lower. Milwaukee and St. Louis, on the other hand, showed second peaks higher than the first. The usual phenomenon, however, was a large first wave followed by smaller ones.
The highest, or maximum peak rate of mortality during the epidemic varied greatly, from 31.6 per 1,000 in the case of Grand Rapids, to 158.3 per 1,000 in the case of Philadelphia.
The death rates which were of the most frequent occurrence were, generally speaking, rates below 70 per 1,000 per week.
The date of the week in which the maximum peak rate occurred was earliest in Boston and Cambridge, where it occurred October 5th, and latest in Grand Rapids, Milwaukee and St. Louis (December 14th). Thirty-one of the 40 cities studied had attained the peak rate of mortality prior to November 2d. In the case of Milwaukee and St. Louis the maximum peak was the second peak, whereas in Grand Rapids it was the first peak that was so late. Sixty-five per cent. of the 40 cities showed two distinct peaks in the mortality curve, while 15 per cent. had one peak, and 8 or 20 per cent. had three peaks.
“It appears clearly that there was a definite tendency for the two-peak cities to fall into two groups in respect of the time elapsing between first and second peaks. About a third of them had the second mortality peak around eight weeks after the first peak. The remaining two-thirds had the second peak, on the average, about thirteen weeks after the first. The three-peak curves had the second peak on an average 7.1 ± 0.3 weeks after the first, and the third peak on an average 13.1 ± 0.3 weeks after the second. The cycle in the epidemic waves would therefore appear to be nearly a multiple of seven weeks rather than the ten weeks tentatively deduced from the dates of peaks. There the process of averaging obscured the true relations.”
Duration of explosive outbreak.—The range of the duration of the first outbreak of epidemic mortality is great, varying from five weeks in Richmond, Virginia, to twenty-three weeks in Atlanta, Georgia. Twenty of the cities, one-half the total number, showed a duration of ten weeks or less, while in the other half the duration was eleven weeks or more. The mean duration of epidemic mortality in the first outbreak was 11.90 ± 0.55 weeks. The ascending limb of mortality rate was rapid in nearly all cities. The descending limb was usually slower. In 34 of the 40 cities it required four weeks or less time for the mortality rate to pass from normal to its epidemic peak. But in only half as many (17) of the cities did the rate come down from its peak to normal again in a period of four weeks or less. The mean time from normal mortality rate to peak was 3.90 ± 0.21 weeks. The mean time from peak mortality rate to normal was 8.00 ± 0.50 weeks. Thus it took about twice as many weeks for the mortality curve to come back from its peak to normal, as were required for the increase from normal to peak at the beginning of the explosion. This is on the average. The ascending limb occupied about a month and the descending limb two months.